In computed radiography (CR) systems, an X-ray image is produced by exposing a special storage-phosphor screen to X-rays which have been attenuated by passing through an object (usually a portion of the human body) to be imaged. The image is captured in the storage-phosphor screen. The storage mechanism involves exciting the electrons in the atoms comprising the screen to higher meta-stable energy states. The energy state varies depending on the amount of attenuation experienced by the X-rays as they pass through the body. Each electron, being in a meta-stable state, can be stimulated back to its ground state by the application of a sufficient excitation energy, and in doing so emits a photon of visible light, usually blue.
Thus, the stored image is "read" from the screen by scanning the screen with a laser beam of a specific wavelength. The emitted photons can be detected by a detector, and by blocking the stimulating wavelengths of the laser from the detector, the detected signals can be processed to reconstruct the image.
It is important that the CR system be properly calibrated and tested on a regular basis to ensure an accurate rendering of images. Testing and adjusting the image quality performance of the imaging sub-system typically requires imaging one or more test patterns and inspecting the readout image. Performance indicators of interest include: resolution, geometric accuracy, uniformity, signal accuracy and added noise. Resolution performance can be measured with a variety of commercially available resolution patterns. Typically these resolution patterns consist of arrays of thin lead strips of varying widths at varying intervals. Signal accuracy can be measured by various step wedge phantoms which provide varying amounts of X-ray attenuation. Uniformity can be measured by evaluating uniformly exposed portions of the image. Geometric accuracy requires imaging a radio opaque pattern of known dimensions. While a small object such as a circle or a square could be used to measure the aspect ratio of the image, measuring the geometric image over the entire image would require a test pattern equal to the size of the screen.
While many prior art test systems and methods are known, it is interesting to note that such systems involve variations of a central thesis, namely imaging a phantom, or some test object, and evaluating the resulting read-out image. For example U.S. Pat. No. 5,056,130 describes a calibrator comprised of a set of interchangeable pieces. The pieces have known different densities, thus permitting the assembly of configurations of known size and densities for calibrating a computerized tomographic system.
U.S. Pat. No. 5,095,431 discloses a calibration method for an X-ray scanner. The method includes positioning a non-circular shaped standard between the X-ray source and a plurality of detectors. A series of attenuation measurements are made for each of a number of principle angular positions of the standard. The resulting data is then analyzed to produce calibration curves of the X-ray system.
U.S. Pat. No. 5,236,363 describes an improved phantom having two X-ray attenuation regions, one region formed within the other. The phantom is capable of simulating a human bladder (inner region) that is surrounded by pelvic bone (outer region).
U.S. Pat. No. 5,539,799 discloses a test measurement body (2, FIGS. 1 and 2) for acceptance and stability testing of dental radiographic equipment. The test measurement body includes absorption elements (14) having varying X-ray absorption characteristics. Radiation from the X-ray source penetrates the test body and is detected by a sensor (3) which converts the detected radiation into electrical signals. The signals are fed into a computer (4) which then analyses the signals for deviations from a reference.
U.S. Pat. No. 5,544,157 describes a calibration template for producing standardized X-ray images. The template comprises an enclosure having materials of various shapes and densities to simulate the X-ray absorption properties of the human body, including bone, organs and other soft tissues.
These prior art systems rely on an X-ray source and a storage-phosphor screen as the image-producing components of a diagnostic procedure for testing and adjusting the imaging system. As such the quality and accuracy of the test is dependent on the condition and performance of these sub-systems. For example, test results are affected by X-ray dose accuracy and uniformity. The X-rays add quantum noise to the image. The image is dependent on the particular X-ray technique, which affects the X-ray energy and the amount of scatter. Finally, variations between X-rays sources affect uniformity of the tests.
The storage-phosphor screen also affects the test results. The construction of the screen itself affects resolution, noise and signal strength. Results vary among screens from different manufacturers and sometimes from the same manufacturer, even among different samples of the same screen model. Imperfections such as scratches and smudges on the surface cause artifacts which can affect the results. Finally, since the scanning process partially erases the stored image, each image must be exposed then scanned once and erased before the next image can be made, a time-consuming process that does not lend itself to repetition which makes adjustments and troubleshooting difficult to implement.
What is needed is a means of generating images for a CR system that does not involve the use of X-rays. It is also desirable to eliminate the storage-phosphor screen in order to eliminate certain variations inherent in their physical construction. What is needed therefore is apparatus which allows adjusting and testing of a CR system that avoids the potential inaccuracies and variations caused by the X-ray sub-system and storage-phosphor screen.